The two numbers that matter

Paternity tests produce two types of results, and each has a different accuracy standard:

  • Inclusion (probability of paternity): When the alleged father could be the biological father, the test reports a probability, typically 99.99% or higher. Some labs report 99.999%.
  • Exclusion: When the alleged father is not the biological father, the test is 100% definitive. There's no statistical wiggle room. The DNA either matches the required patterns or it doesn't.

Quick fact: 100% accuracy on exclusions isn't a marketing claim. It's how the underlying science works. If two non-shared STR markers don't match, the alleged father cannot be the biological father. Full stop.

What "99.99%" actually means

The 99.99% number is a calculated probability of paternity, comparing the likelihood that the tested man is the father versus a random untested man from the same population.

For an inclusion, the lab compares the child's DNA markers against the alleged father's. If they match, the lab calculates how rare those matching markers are in the general population. The rarer the combination, the higher the probability number.

A 99.99% probability of paternity means it's 9,999 times more likely the tested man is the father than a random man would be. For all practical purposes, that's definitive.

What "99.99%" doesn't mean

A few things people often get wrong:

  • It doesn't mean 0.01% chance someone else is the father. That's not what the math measures.
  • It doesn't account for siblings or close relatives who weren't tested. If the alleged father has an identical twin, only direct testing of both can distinguish them.
  • It doesn't mean all tests achieve 99.99%. The number depends on how many STR markers are tested. Most accredited labs test 20+ markers, but some cheap home kits test fewer.

Why accreditation matters

Not all DNA labs are equal. The gold standard is AABB accreditation (formerly American Association of Blood Banks). AABB-accredited labs:

  • Test a minimum number of STR markers (typically 20+)
  • Follow strict quality control protocols
  • Are subject to regular audits
  • Produce results admissible in U.S. courts

A test from an AABB-accredited lab is the only kind that meets the standards for court use, child support cases, custody disputes, immigration filings, and birth certificate amendments. Home kits and non-accredited labs may produce a result, but it carries no legal weight.

Can a DNA paternity test be wrong?

From accredited labs, errors are vanishingly rare and almost always trace to human factors, not the science:

  • Sample swap or contamination during collection (avoided through chain-of-custody protocols)
  • Wrong person being tested (avoided through ID verification for legal tests)
  • Lab error (rare, but accredited labs run controls and confirmation tests to catch these)

For peace-of-mind tests (where you collect samples yourself), the most common "error" isn't a lab error at all. It's mistaken identity. The person you think you swabbed might not be who you think, or samples may have gotten mixed up at home. For absolute certainty, chain-of-custody legal tests eliminate these variables.

Key Takeaway

A DNA paternity test from an AABB-accredited lab is 99.99%+ accurate for inclusions and 100% definitive for exclusions. The science is settled. Choose an accredited lab partner, and the result you get is the truth.